1. Field of the Invention
This invention relates to a scanning optical apparatus and an image forming apparatus using the same, and particularly is suitable for an image forming apparatus such as a laser beam printer or a digital copying machine adapted to effect image formation by the use of a multi-semiconductor laser having a plurality of light emitting points as light source means to achieve a high speed and high recording density.
2. Description of Related Art
FIG. 6 of the accompanying drawings is a cross-sectional view of the essential portions in the main scanning direction (main scanning cross-sectional view) of a conventional multi-beam scanning optical apparatus using light source means having a plurality of light emitting points.
In FIG. 6, two beams (light beams) emitted from a multi-semiconductor laser 51 having two light emitting points 51A and 51B are converted into substantially parallel beams by a collimator lens 52, and the substantially parallel beam is converged only in a sub scanning direction by a cylindrical lens 54 having predetermined refractive power only in the sub scanning direction.
Further, the difference in aberration occurring from the difference between the beams tracing from the two light emitting points 51A and 51B separate from an optical axis affects the imaging performance and therefore, by an aperture stop 53 for reducing the influence thereof to the utmost, the beams are shaped and imaged into the shape of a focal line extending long in the main scanning direction near the deflecting surface (reflecting surface) 55c of a polygon mirror 55 which is a light deflector.
The two beams reflected and deflected by the polygon mirror 55 being rotated at a constant angular speed in the direction of arrow 55b are condensed into spot shapes on a photosensitive drum surface 57 as a surface to be scanned by two fθ lenses 56a and 56b as a scanning lens system 56, and are scanned at a constant speed in the direction of arrow 57b (the main scanning direction).
In such a multi-beam scanning optical apparatus, if as shown in FIG. 7 of the accompanying drawings, the two light emitting points 51A and 51B are arranged rectilinearly in the sub scanning direction, the interval between the two scanning lines in the sub scanning direction on the photosensitive drum surface becomes greater than recording density and therefore, usually as shown in FIG. 8, the two light emitting elements 51A and 51B are disposed obliquely with respect to a direction corresponding to the sub scanning direction, and by adjusting the angle of inclination θ thereof, the interval between a plurality of scanning lines in the sub scanning direction on the photosensitive drum surface is accurately adjusted in accordance with the recording density.
By doing this, it becomes possible to make the paths of the light beams coincident with each other to the utmost to thereby achieve higher recording density, and further the number of revolutions of a driving motor for the polygon mirror can be suppressed to ½ as compared with a case where the number of light emitting points is one, and it becomes possible to cope with a higher speed with a sufficient surplus.
Now, to effect scanning of high density at a high speed, it is likewise necessary to cope with the control of the time when image information starts to be written.
In FIG. 6, there is provided synchronism detecting means (BD optical system) 58 for controlling the timing of a scanning start position on the surface to be scanned, and there are disposed a synchronism detecting device (BD sensor) 59 for detecting a synchronizing signal (BD signal) and a synchronism detecting optical system (BD lens) 60 for directing a synchronism detecting beam (BD beam) to the synchronism detecting device 59.
The beam converged only in the sub scanning direction by the cylindrical lens 54 as previously described is made into a linear beam extending in the main scanning direction near a deflecting surface 58c, and enters the synchronism detecting optical system 60 as a substantially parallel beam in the main scanning cross section and as a divergent beam in the sub scanning cross section.
The synchronism detecting optical system 60 has independent focal lengths in the main scanning cross section and the sub scanning cross section, respectively, and is given power suited to be condensed on the synchronism detecting device 59 in the main scanning cross section and the sub scanning cross section, respectively, and the beam forms a spot on the surface of the synchronism detecting device 59.
In a single beam scanning optical apparatus, contrivance is done to make a semiconductor laser as a light source which is an electrical part and a synchronism detecting device integral with each other, and dispose the light source and the synchronism detecting device on one and the same substrate for the curtailment of the number of parts and cost, while in the case of a multi-beam scanning optical apparatus, writing is effected at a time at intervals in the sub scanning direction by a plurality of light sources and therefore, as means for adjusting the intervals, a composite optical system comprising a prism, a mirror, etc. disposed on the light emitting side near the light sources is adjusted. Also, in the case of a so-called monolithic multi-beam scanning optical apparatus having a light source having a plurality of light emitting points, the adjustment for accurately adjusting the pitch interval on a surface to be scanned in the sub scanning direction as by rotating the light source and a circuit substrate on which the light source is disposed is necessary.
Therefore, if the synchronism detecting device remains disposed on the same substrate as the light source, the synchronism detecting device will be moved with the rotation of the light source. When such structure as shown, for example, in FIG. 9 of the accompanying drawings wherein the form in the case of a single beam is intactly utilized and is rotated about a light source is adopted, in this structure, the light condensing point may deviate from the synchronism detecting device during the rotation, and this lead to the problem that even if the synchronism detecting device is rotated, a sufficient movement range cannot be secured, and when the light condensing point deviates, the work of shifting the location itself of the synchronism detecting device or the light source as by bending the synchronism detecting device or the leg of the light source becomes necessary and it is difficult to effect synchronism detection accurately.
Therefore, the light source and the synchronism detecting device are disposed on discrete substrates and adjustment thereof is effected discretely to thereby cope with this problem, but this leads to a factor for an increased cost because of an increase in the number of parts and the complication of an electric circuit caused by the plurality of substrates, and an increase in the work of arranging the wiring, and improvements have been desired.